Transparent coatings from sol-gel techniques that closely approximate inorganic glasses are commonly formed from alcohols or other non-aqueous solvents. For example, to obtain a continuous transparent coating, a TiO2 glass is often formed from condensation of a tetraalkoxytitanate in the presence of a chelating agent in an alcohol solvent or under restricted conditions, such as synthesis in a glove box. Aqueous solvents, or even using a large quantity of water during the process, generally promote the condensation of precursors into particulate glasses. Even when non-aqueous solvents are used, the formation of a hard robust coating is problematic as the resulting glasses often have a tendency to crack because of shrinkage induced stresses upon evaporation of solvents and the loss of condensation byproducts. Because of this propensity for cracking, coating thicknesses in excess of 1.5 μm generally require that multiple thin coating layers are made, usually with practical limits of 20 to 30 coats. The formation of thick single layer coatings is often achieved in a non-aqueous system by the use of an inorganic/organic composite, an organically modified ceramic, where an organic component is included in a colloidal sol-gel system. Generally there is little interpenetration of these inorganic and organic portions, and high hardness with optical transparency is generally not achieved in such systems.
As the use of organic polymer based devices, such as LCD displays and LED lighting, increases, there is a greater need for thick superior abrasive resistant transparent coatings that have excellent barrier properties for plastic or other organic substrates, and where the processing can be carried out with the formation of a single coating layer in a manner that does not damage the underlying substrate. Hence the formation of a transparent hard coating with high solids that act as an excellent diffusion barrier for an underlying substrate remains a need.